This invention relates to control systems, and more specifically to an electronic control system that is capable of being cooperatively associated with a roller mill for purposes of controlling the rate of feed of material to the mill in accordance with the output being demanded from the mill, while yet at the same time ensuring that during changes in the output being demanded from the mill both a constant fineness of pulverized material and a constant air-to-solids ratio from the mill are maintained.
There has long existed a need to effectuate the grinding, i.e., pulverization, of various kinds of materials. To this end, there has been provided in the prior art an assortment of different types of apparatus, each alleged to be suitable for use for the purpose of accomplishing the grinding, i.e., pulverization, of one kind of material or another. One such type of apparatus, which has frequently been utilized for this purpose, is that commonly referred to by those in the industry as a roller mill. The basic operating principle of the roller mill is quite simple. Namely, in accord therewith material is reduced in size as a result of being ground by rolls running centrifugally against a stationary. outer ring.
Continuing, the mode of operation of the roller mill is such that the material, which is to be ground, is introduced into the mill at a controlled rate by feeder means, the latter being cooperatively associated with the mill. Upon entering the roller mill the material to be ground falls to the bottom of the mill. Thereafter, rotating plows which are set at an angle in front of the rolls cause the material to be scooped up in a continuous stream such that the material passes between the rolls and the grinding ring whereupon the material is pulverized through the coaction of the rollers and the ring.
In addition, a large volume of air enters into the roller mill through tangential ports suitably provided for this purpose in the base immediately under the grinding ring. This air serves to sweep the fine and medium fine fractions of the pulverized material into the separating zone. The latter zone is located directly above the grinding elements, i.e., the rolls and the grinding ring.
Within the separating zone, the ground material is classified by a separator. More specifically, as a consequence of this classification, oversize material is made to automatically drop back into the grinding zone whereupon it is subjected to being reduced further in size. On the other hand, the fine material that meets the desired size specifications is carried to a suitable collector for subsequent discharge. Lastly, the cleaned air is made to continuously return to the mill through an exhauster, the latter being suitably connected for this purpose in closed circuit relation with the mill.
A high degree of efficiency is capable of being achieved with the roller mill by virtue both of the fact that the pulverized, i.e., ground, material is removed from the grinding zone promptly and of the fact that the finished product is swept away with the airflow for collection. The result thus is that on the one hand a minimum of excessive fines are produced while on the other hand and concomitantly therewith maximum mill capacity and economy are also being realized.
Notwithstanding the fact that as has been discussed above prior art forms of roller mills have been advantageously characterized in certain respects insofar as concerns both the nature of the construction and the mode of operation thereof, there has nevertheless still existed a need to effect improvements with regard thereto. More specifically, there are several variables relating to the operation of a roller mill, which are known to affect the roller mill's output and/or the fineness of the pulverized material which is produced as a product from the roller mill. In this connection, attention is directed to the following list of variables: feed rate; mill speed; mill pressure drop, i.e., feed retention time in the mill; classifier setting in those instances wherein the roller mill is equipped with a classifier of the static type and classifier speed in those instances wherein the roller mill is equipped with a classifier of the dynamic type; and mill airflow.
Continuing, the aforedescribed variables are known to bear certain relationships one to another. Namely, mill output is known to increase as a direct function of feed rate, mill speed and airflow, and inversely as a function of classifier setting or classifier speed, i.e., higher fineness, depending on the type of classifier with which the roller mill is equipped. On the other hand, product fineness increases directly as a function of mill speed, pressure drop, classifier setting or classifier speed, depending on the type of classifier with which the roller mill is equipped, and inversely as a function of airflow.
The conventional practice heretofore has been to operate roller mills at a constant optimum mill speed, pressure drop and airflow. As such, it has been possible to produce from the roller mill an output having different product finenesses simply by effecting changes in the setting of the classifier or in the speed of the classifier as the case may be depending upon the type of classifier with which the roller mill is equipped. Further, to effect changes from the optimum in any of the other variables, i.e., mill speed, pressure drop or airflow has been found not to be feasible. Namely, it has been found that changing any of these other variables, i.e., mill speed, pressure drop or airflow, from the optimum usually results in a noisy, vibratory mill, which can ultimately cause mechanical failure of the roller mill. In summary, it can thus be seen that prior art forms of roller mills have been disadvantageously characterized in that such mills have proven to be in essence one capacity machines, i.e., machines possessing no turndown capability. This lack of turndown capability can be attributed largely to the fact that insofar as roller mills are concerned there exists a need for an adequate cushion to always be present of the material that is to be ground between the rolls and the grinding ring. Moreover, because of this lack of turndown capability which has served to characterize the operation of prior art forms of roller mills, there has existed a requirement for effecting the storage of the excess portion of the output of the roller mill until such time as a need therefor exists. On the other hand, the storage of ground material for any significant period of time is for a number of reasons acknowledged by those who are skilled in this art to be undesirable.
Attempts are known to have been made to provide a roller mill, which insofar as output capacity is concerned would have a turndown capability. Moreover, one such attempt forms the subject matter of U.S. Pat. No. 4,184,640. In accord with the teachings of the latter U.S. patent, it is proposed to provide a roller mill with a control system wherein the control system is connected in circuit relation with prime mover means, which in turn are connected to the grinding rolls and the classifier means, respectively, of the roller mill. With further reference to U.S. Pat. No. 4,184,640, as described therein the mode of operation of the control system is such that the latter is effective to vary the speed of the prime mover means that is connected to the grinding rolls as well as that of the prime mover means that is connected to the classifier means whereby the particle size, i.e., fineness, of the material being discharged from a roller mill equipped with the subject control system is maintained substantially uniform through adjustment of the centrifugal force that the grinding rolls exert inversely with the speed of the classifier means.
Although the control system which forms the subject matter of U.S. Pat. No. 4,184,640 is alleged, when cooperative1y associated with a roller mill, to be operative to obviate the problem which heretofore has served to plague prior art forms of roller mills, i.e., the fact that prior art forms of roller mills did not possess any turndown capability insofar as output capacity is concerned, it has nevertheless served to introduce a new and different problem. Namely, it has been found that a characteristic of a roller mill equipped with a control system constructed in accordance with the teachings of U.S. Pat. No. 4,184,640 is that such a roller mill exhibits a susceptibility to being subjected either to a condition wherein the material to be ground in the roller mill is fed thereto at an excessive rate whereupon the roller mill can become plugged or to a condition wherein the material to be ground in the roller mill is fed thereto at an insufficient rate whereupon the mill can become underfed. The way in which this comes about is that should the demand for output from the roller mill change significantly the control system constructed in accordance with the teachings of U.S. Pat. No. 4,184,640 is designed to cause an appropriate change to occur both in the speed of the grinding rolls and in the speed of the classifier means. This in turn has the effect of producing a variation in the differential pressure measured relative to the roller mill. Further, as a consequence of the existence of this variation in the differential pressure, a change will be had in the rate at which the material to be ground in the roller mill is being fed thereto.
One can, thus, see that in accord with the mode of operation of the control system constructed in accordance with the teachings of U.S. Pat. No. 4,184,640, although a change in the demand for output of the roller mill occasions, relatively speaking, an immediate change in the speed of the grinding rolls and in the speed of the classifier means, it does not produce any such immediate change in the rate of feed of material to the roller mill until such time as the change in the speed of the grinding rolls and in the speed of the classifier means causes the differential pressure measured relative to the roller mill to vary. Moreover, it is only when this variation in differential pressure becomes measurable that the rate of feed of material to the roller mill is affected. Unfortunately, by the time this occurs, the rate of feed of material to the roller mill will continue at the same rate notwithstanding the fact that the demand for output from the roller mill may have diminished sharply or may have increased sharply which thereby in turn may in the case of the diminished demand cause the roller mill to become plugged by virtue of the fact that the rate of feed of material to the roller mill is being maintained at an excessive rate of feed as compared to that required to meet the diminished demand, or in the case of the increased demand may cause the roller mill to become underfed by virtue of the fact that the rate of feed of the material to the roller mill is being maintained at an insufficient rate of feed as compared to that required to meet the increased demand for output from the roller mill. Either of these two conditions, i.e., an overfeeding of the roller mill or an underfeeding of the roller mill, can cause the roller mill to suffer severe damage. Accordingly, it is desirable to provide a roller mill which embodies a mode of operation wherein both of these two conditions, i.e., an overfeeding and an underfeeding of the roller mill, are avoided.
A need has thus existed in the prior art for a new and improved control system with which a roller mill might be equipped, and which would function to control the mode of operation of the roller mill. Moreover, a need has been demonstrated for such a control system which when cooperatively associated with a roller mill would be operative to provide the roller mill with a turndown capability insofar as relates to the matter of the latter's output capacity. Finally, a need has been shown for such a control system which would prevent a roller mill equipped therewith from suffering from the same difficulties that have served to disadvantageously characterize the operation of roller mills that embody. either no control system for controlling the mode of operation thereof or a control system which may cause the roller mill to become overfed or underfed.
It is, therefore, an object of the present invention to provide a new and improved control system suitable for employment in cooperative association with a roller mill, and operative to effectuate control over the mode of operation of the roller mill so equipped therewith.
It is another object of the present invention to provide such a control system that is operative when cooperatively associated with a roller mill to provide the latter with a turndown capability insofar as concerns the output capacity thereof, i.e., to enable the roller mill to operate over a wide range of output capacities.
It is still another object of the present invention to provide such a control system that is operative when cooperatively associated with a roller mill to enable the latter to operate over a wide range of output capacities while yet permitting a constant product fineness to be maintained despite the fact that the roller mill may be required to operate at different rates of output capacity.
A further object of the present invention is to provide such a control system that is operative when cooperatively associated with a roller mill to enable the latter to operate over a wide range of output capacities while yet permitting a constant air-to-solids ratio to be maintained from the mill, within the restraints of air-to-solids conveying pipe velocities, despite the fact that the roller mill may be required to operate at different rates of output capacity.
A still further object of the present invention is to provide such a control system that is operative when cooperatively associated with a roller mill to enable the latter to operate over a wide range of output capacities in such a manner that there exists a feed forward capability. whereby changes in the rate of feed of material to the mill can be effected when the demand for output from the mill changes without in turn producing a condition which may cause either an overfeeding of the mill or an underfeeding of the mill.
Yet another object of the present invention is to provide such a control system which when cooperatively associated with a roller mill is operative to control the mode of operation of the roller mill, and which is characterized in that a different constant for product fineness and/or a different constant for the air-to-solids ratio may be selectively introduced into the control system in order to satisfy the need therefor arising out of the fact that there exists a requirement to operate the roller mill in accordance with different operating parameters such as those which by way of exemplification and not limitation might be occasioned by a change in the nature of the characteristics of the material that is to be ground in the roller mill.
Yet still another object of the present invention is to provide such a control system operative when cooperatively associated with a roller mill to control the mode of operation of the roller mill which is relatively simple to construct and employ, as well as being relatively inexpensive to provide.